High energy accelerators for particle beams are used more and more throughout the world. In doing so, intensity and energy are increased permanently. For instance, currently proton accelerators with energies up to the range of tera-electron volt (TeV) are planned and proton accelerators with energies up to some giga-electron volt (GeV) and intensities up to 1016 protons/sec are planned, e.g. for spallation sources.
The latter accelerators are not only planned as neutron sources for fundamental research, but are also discussed as nuclear facilities for energy production, by which subcritical systems can be brought into a critical state by an additional neutron flow. Furthermore, those facilities can be used for the so-called incineration, during which long-lived radioactive substances are changed into short-lived ones.
When running high energy accelerators, one problem is the production of high-energy secondary radiation in the target areas (Target of the particle beam, in which it is deposited) or in case of beam losses during the transport on the path of the beam guidances of the high energy or primary beam to the target.
While the charged particles generated in nuclear reactions are often stopped in the structure of the accelerator, the generated neutron and gamma radiation has a high capability for permeating, even through shieldings with a thickness of some meters. Furthermore, at very high energies inter alia pions are generated, which decay into muons. Latter have also a very high range and have therefore to be stopped in special beam annihilators.
In case of heavy ion accelerators the situation is yet more difficult, because already at lower intensities similar production rates for secondary radiation arise, compared to proton accelerators. So far, the production of radiation at such accelerator facilities caused the installation of mostly very massive shieldings at the places of beam losses.
Often iron or concrete was used as shielding material like in nuclear technology. Such concrete shieldings consist of hard-casted walls and ceilings, but also single shielding modules assembled from single parts can form an overall shielding.
For special shielding requirements heavy varieties of concrete with appropriate additives like magnetite, limonite or barite, concrete with densities up to 3.6 g/cm3 can be used besides normal concrete with a density in the range of 2.3 g/cm3 (see also Deutsche Industrienorm DIN 25413). But in practice, normal concrete is mostly used in the sense of optimizing cost and attained shielding result.
Producing the radiation depends on the kind of radiation, the energy, the intensity and the loss rate. Furthermore, the shielding thickness depends on limit values to be met according to the national legislations. The limit values are defined as annual dose limit values or are referred to the dose rate in μSv/h.
Recently, using shielding arrangements with bulk material was proposed. For instance, gypsum or iron ore were proposed as bulk material. Though being naturally occurring material was heaped up around these facilities as soil up to now, but not incorporated directly into the shielding. On the other hand, the problem of activation arises, when natural material is used in the shielding arrangement, because this material is relatively close to the sources.
From the patent applications DE 103 27 466 (Forster) and DE 103 12 271 A1 (Brüchle et al.) gypsum is known as alternative material for parts of a radiation protection structure and the shieldings of high energy accelerators respectively. This material proved to be well suited as shielding material, too.
Using such shieldings, which have bulk material as shielding substance, implicates some enhancements, but the previous developments and proposals to construct shieldings for accelerator facilities have mostly been planned in particular consideration of the shielding properties.
A further effect addressed by the present invention, being important and due to the inventors' findings not being sufficiently considered so far is the activation of the radiation protection material itself, particularly the generation of radioactivity by secondary radiation, which causes nuclear reactions in the shieldings. In these unwanted side-effects the generation of radionuclides is particularly caused in spallation reactions by protons and is neutrons in the shielding layers. A plurality of radionuclides can be generated by evaporation of nucleons and clusters. This problem is yet deteriorated by the fact that the heavier the target nucleus of the used shielding material is, the greater the variability of the generated radionuclids becomes.
If natural material, which should be recirculated to a natural utilization after termination of using the facility, is used for shielding purposes, the level of the generated radioactivity has to go below certain limits in order to comply with the specifications of the national legislation. So, for example, one has to go below under a nuclide-specific approval value Ai in Bq/g for the unlimited release according to German radiation protection law. In case of several radionuclides the total exhaustion after applying the sum rule has to be less than one. The total exhaustion is defined as:
      G    =                  ∑                  i          =          1                max            ⁢                        A          i                          F          i                      ,Where Fi is the real activity per mass and radionuclide and where one has to be sum up over all radionuclides (i).
According to German law there is still a further limit value for the restricted release beside the unlimited release (able for being deposited), but irrespective of potential legal limit values, an activity is desirable, which is as low as possible.
Calculations by the inventors, however, showed that, when operating a high energy accelerator facility at very high intensities over several decades, the used shielding material is activated so highly that it is not able for being cleared after switching off the facility and in the deconstruction phase, not even for restricted release as the case may be, and it has to be stored for years or decades before it can be released. This applies also for natural filler material (soil, sand, water etc.), which is used just for the reason to be recirculated to a natural utilization as soon as possible after terminating the using of the facility. But if its exhaustion is above the legal limits, this object cannot be met, because the material would have to be stored intermediately or would have to be disposed with enormous costs as radioactive waste.
From the patent application DE 103 27 466 A1 a structure with a sandwich construction method for a radiation protection building is known. This structure, however, comes from a room for medical proton treatment, whose requirements are not comparable, because of the essentially lower energies.
Summarizing, especially multi-layered radiation protection arrangements or walls for high energy accelerator facilities have to be further improved with respect to the radioactive activation of the material and its deactivation properties, in consideration of operating over several years or decades with high beam energies and intensities and the disposal thereafter. Particularly, this aspect is of special importance, if natural shielding material is used, which on the one hand is radioactively activated after having operated the facility and on the other hand there is few experience in handling higher quantities of such material.